Automotive vehicles often employ mirrors including a convex surface which is designed to provide a compromise between several design objectives. These objectives typically include a field of view, mirror size and location, apparent image size, and distortion.
My U.S. Pat. No. 4,449,786 discloses a mirror having a convex curvature defined by mathematical and geometrical relationships between the position of the observer, the viewed objects, and the mirror. The design criteria which I disclose in my '786 patent yield mirror surfaces which control bi-ocular distortions while producing the desired field of view. My patented design provides an improved mirror surface which typically includes a relatively flat, spherical-convex primary viewing area combined with a peripheral, aspherical-convex viewing area employing a continuously decreasing radius of curvature (and, generally a continuously increasing field angle) as the surface extended outwardly from the central or primary viewing area of the mirror. This concept is currently successfully employed on about 50,000 light-truck vehicles in North America. While my patented design defines a surface having a smooth optical transition between the primary viewing area and the more radically curved aspherical peripheral area, the design objectives for some mirrors often present problems to the mirror optics designer and/or manufacturer.
For example, on mirrors with relatively limited width and/or large field-of-view requirements, a relatively greater portion of the mirror profile must be provided with an aspherical convex surface to obtain the desired field of view from the mirror. This often results in rapidly decreasing radii of curvature of the mirror's surface at the periphery of the mirror, as well as at the periphery of the corresponding mold surface. While often necessary to obtain the desired view angle, this rapidly increasing curvature in the peripheral area provide increased and unwanted distortion of reflected images in that area.
Another problem arises in the manufacture of mirrors having a relatively small or rapidly decreasing radii of curvature at their periphery. The glass for these mirrors is typically formed on glass bending equipment which heats the mirror glass to its softening point, and bends the softened glass, either by the force of gravity over a mold, or by press-forming the glass sheet between two complementary molds. In mirrors having designs which include relatively rapidly decreasing radii of curvature at their periphery, the sharp curvature of the glass at the periphery may prove difficult to form on existing glass bending equipment without heating the glass to a higher temperature in the areas of sharper curvature and/or other more expensive and time-consuming forming methods and equipment. As is appreciated by those skilled in the art, heating glass to an increased temperature to form relatively sharp bends in a glass often negatively affects the surface quality of the glass, thereby yielding mirror lenses of low or unacceptable quality. This problem is exacerbated by the fact that the mold surface is necessarily larger than the finished mirror, often resulting in a forming surface which, at its periphery, has a more severe curvature than the smaller finished mirror formed thereon.
One object of the present invention is, therefore, to provide an improved vehicle mirror design which balances the considerations of limited viewing surface with wide field of view while simultaneously minimizing both (1) the image distortion caused at the transition points between changing viewing surface profiles and (2) the image distortion resulting from relatively rapidly decreasing radii of curvature at the periphery of the mirror.
Another object of the present invention is to provide an improved method for designing a vehicle mirror surface which yields a controlled image distortion and a wide field of view while minimizing the amount of image distortion at the periphery of the mirror.
It is yet another object of the present invention to provide a method of designing a mold surface for fabricating a vehicle mirror which provides a resultant mirror having a surface which provides both controlled image distortion and a wide field of view while minimizing the sharpness of bend required in the mold surface at and nominally beyond the periphery of the mirror.